The present invention generally relates to the field of refrigeration systems and, more specifically, to a cascade refrigeration employing a peak control feature for monitoring and limiting the discharge pressure, and therefore the discharge temperature, of the low stage compressor in a two stage system.
Until recent years, chlorinated fluorocarbon (CFC) refrigerants had been widely used in a variety of refrigeration systems. Due to environmental concerns of such products, however, there has been a move to replace CFC's with other, more environmentally safe, refrigerants. In two stage cascade refrigeration systems, for example, the CFC refrigerant previously used in the high temperature stage part of the system may be replaced by the more environmentally safe R-134a without a significant adverse effect on the system. Low temperature stage CFC refrigerants such as R-503 are less easily replaced and, in fact, only one replacement, R-23, is currently available and approved for use in cascade refrigeration systems of the type contemplated herein.
Certain problems have arisen as a result of the transition to environmentally safe refrigerants. Most notably, in the low temperature stage portion of a cascade refrigeration system the replacement refrigerant has caused an unacceptable rise in compressor discharge pressure and temperature. This is especially true during start-up conditions and other high load situations, such as during high ambient temperature conditions. This reduces the efficiency of the system, i.e., makes the system work harder to achieve the desired temperature, and also adversely affects components within the low temperature stage compressor. In this latter regard, for example, typical compressors used in the low temperature stage have outlet valves which are designed to withstand temperatures up to about 160-165 degrees Celsius. Temperatures will exceed this maximum at a compressor discharge pressure of about 155 psig and above. With the use of the replacement refrigerant R-23, compressor discharge temperatures have been found to increase and easily exceed the temperature range necessary for preventing compressor outlet valve overheating and failure.
The above-mentioned problems in the art have not been adequately addressed during this changeover from CFC refrigerants to environmentally safe refrigerants. Certain proposed refrigeration or air conditioning systems, such as those disclosed in U.S. Pat. Nos. 4,841,739 and 5,259,204 have addressed the environmental hazards of CFC refrigerants by attempting to prevent blow-off of the refrigerant from the system into the atmosphere. Other systems have used pressure control features in attempting to increase the efficiency of system. However, none of these systems have addressed the problems presented by the use of newer, more environmentally safe refrigerants.
There is a need in the art, therefore, for a system which is able to use environmentally safe refrigerants while maintaining the desired system efficiency and also maintaining a long useful compressor life especially under adverse or high load operating conditions.